1. Field of the Invention
The present invention relates to a circuit for reading a line-transfer photosensitive device. The invention is also concerned with a line-transfer photosensitive device incorporating said reading circuit and with a method for reading said device.
2. Description of the Prior Art
Row-transfer or so-called line-transfer photosensitive devices are well-known in the prior art. It is recalled that a device of this type as represented schematically in FIG. 1 of the accompanying drawings usually has a photosensitive zone 1 consisting of a matrix array of M "lines" or rows each composed of N photosensitive elements P. This zone receives the image to be scanned and converts it to electric charges or so-called signal charges Q.sub.s. The photosensitive elements of one and the same row are connected to each other as well as to an address register 2 which serves to select one row of the matrix. The photosensitive elements of any one column are connected to the same conductive column 3. When one row of the matrix is selected by the address register, the signal charges created within each of the photosensitive elements of this row are transferred via conductive columns 3 to a charge-coupled read register 4 having parallel inputs and a series output.
European patent Application No. 0,078,038 filed in the name of Matsushita and French patent No. 2,538,200 granted to Thomson-CSF relate to line-transfer photosensitive devices.
In these two patents, efficiency of charge transfer is enhanced by superimposing a quantity Q.sub.o of a so-called drive or polarization charge at the time of transfer of each signal charge quantity Q.sub.s from a conductive column 3 to a capacitor C.sub.1, then by superimposing a quantity Q.sub.1 of drive or polarization charge at the time of transfer of each signal charge quantity Q.sub.s from a capacitor C.sub.1 to the read register.
It is known that, when charges are transferred by skimming above a potential barrier from a source capacitor to a drain capacitor, it is necessary to superimpose on the signal to be transferred a constant polarization charge which maintains efficiency of transfer at an acceptable level irrespective of the amplitude of the signal to be transferred.
The different transfers considered in the foregoing are illustrated in FIG. 2. These transfers constitute only part of the transfers described in the cited patent to Thomson-CSF in which transfer of parasitic charges is also carried out with polarization charges.
When carrying out transfers from the capacitors C.sub.1 to the read register, the drive-charge quantities Q.sub.o are retained in the capacitors C.sub.1 and charge quantities equal to Q.sub.1 +Q.sub.S are transmitted into the register. Charge quantities equal to Q.sub.1 +Q.sub.S are therefore read at the output of the register.
After each signal charge transfer Q.sub.s, polarization charges Q.sub.o and Q.sub.1 are returned respectively from the capacitors C.sub.1 to the columns and from the read register to the capacitors C.sub.1 as illustrated in FIG. 2.
The problem which arises is that the structures proposed in the prior art can no longer be used when the conductive columns have a high capacitance of the order of one nanofarad as is the case in current applications which will be considered in detail hereinafter.
The object of the description which now follows will be to show the limits of structures proposed in the prior art in regard to the maximum permissible value of capacitance of the conductive columns.
The capacitance of a charge-coupled register is such that the quantity of drive or polarization charge Q.sub.1 employed cannot exceed a few picocoulombs without resulting in a register 4 having unacceptable surface areas. The register must in fact be capable of transporting the drive charge Q.sub.1 as well as a signal charge Q.sub.S having a maximum value of a few picocoulombs. By way of example, the following limit will be adopted in regard to the value of Q.sub.1 : EQU Q.sub.1 .ltoreq.1 pC (1)
Moreover, as disclosed in the cited patent to Thomson, for example, it is known that a quantity of drive charge Q.sub.i must have a sufficient value to changeover to high inversion at the commencement of transfer of charges produced by a capacitor C.sub.i. This condition is represented by the following formula: ##EQU1## where: .phi..sub.F gives the position of the Fermi level,
k is the Boltzmann constant, PA1 T is the temperature PA1 q is the charge of the electron, PA1 N.sub.D is the dopant concentration of the substrate, PA1 n.sub.i is the intrinsic concentration.
There are chosen for N.sub.D and n.sub.i the following mean values: N.sub.D =10.sup.16 /cm.sup.3 and n.sub.i =10.sup.10 cm.sup.3, which gives the following condition in regard to the values of Q.sub.1 and C.sub.1 : ##EQU2##
Taking into account relation (1), the following condition is obtained in regard to the value of the capacitor C.sub.1 : ##EQU3##
Moreover, the capacitor C.sub.1 must be capable of storing the charge quantities Q.sub.o with a voltage swing .DELTA.V which is compatible with the voltage sources usually employed in semiconductors. Since this voltage swing .DELTA.V is of the order of a few volts, the following condition may be established: EQU .DELTA.V.ltoreq.10 V (4)
and the maximum polarization-charge quantity Q.sub.o which can be stored in each capacitor C.sub.1 is written as follows, taking into account relations (3) and (4): EQU Q.sub.o .ltoreq.2.8 pF.multidot.10 V=28 pC (5)
The application of the condition stated earlier in regard to high-inversion transfer: ##EQU4## serves to determine the maximum permissible value of capacitance in the case of the capacitors C.sub.o of the conductive columns 3: ##EQU5##
Relation (6) shows that the structures proposed in the prior art do not permit satisfactory operation when the capacitor C.sub.o of the conductive columns 3 has a value greater than about ten picofarads.
This can be verified by computing the polarization charge Q.sub.o which is made necessary when employing conductive columns 3 having a capacitance C.sub.o of the order of 1000 pF.
The application of the condition stated in the foregoing in regard to high-inversion transfer: ##EQU6## produces the following condition: EQU Q.sub.o .gtoreq.360 pC (7)
Moreover, in the article entitled "Line-transfer image sensor operating in the double-reading mode" by J. L. Berger, L. Brissot and Y. Cazaux of Thomson-CSF and published on Aug. 8th, 1985 in IEEE Transactions on Electron Devices, vol. ED 32, No. 8, there is defined on page 1517 in relation (6) an expression of transfer inefficiency which makes it possible to calculate the value of the drive-charge quantity employed for carrying out this transfer.
Inefficiency of transfer .epsilon..sub.o from the capacitor C.sub.o of the columns to a capacitor C.sub.1 is written as follows when a term of the second order .epsilon..sub.F is disregarded: ##EQU7## where T.sub.1 is the transfer time-duration and characterizes the channel in which the transfer takes place.
The expression of Q.sub.o is accordingly as follows: ##EQU8##
We obtain in respect of Q.sub.o the following value: EQU Q.sub.o =1000 pC (10)
by adopting the following common values: ##EQU9##
The value obtained in the case of Q.sub.o (10) is entirely disproportionate in view of relation (5), Q.sub.o .ltoreq.28 pC, which was obtained in the course of previous calculations.
The present Applicant has therefore shown that the structures proposed in the prior art are unusable when the capacitor C.sub.o of the conductive columns has a high value with respect to the storage capacitance of the photosensitive element and of the read register. For example, when said capacitor C.sub.o has a value of the order of 1 nF, the capacitance of the photosensitive elements and of the register is accordingly of the order of 1 pF.
It must be understood that the structures of the prior art are suitable for use when the capacitance of the photosensitive elements, of the conductive columns and of the charge-coupled register is of the order of a few picofarads.